1. Field of the Invention
The present invention relates to an optical information recording medium for recording and reproducing information with a large capacity, using laser light.
2. Description of the Prior Art
Currently, DVDs can record signals at a higher density than any other read-only optical information recording media commercially available at present.
For a read-only DVD having a diameter of 120 mm, there are specified formats such as a single layer type of reading one face with a maximum user capacity of 4.7 GB, a single layer type of reading two faces with a maximum of 9.4 GB, and a double layer type of reading one face with a maximum of 8.5 GB. FIG. 7 shows an example of the structure of an optical disk of the double layer type of reading one face (hereinafter, referred to as xe2x80x9cone-face-double layer structure). In this optical disk, signals recorded either in a first information layer 21 or a second information layer 23 can be reproduced by irradiating the information layers with laser light 25 through a substrate 20. An optically separating layer 22 for separating the first information layer 21 and the second information layer 23 optically with respect to the incident laser light 25 is provided between the first information layer 21 and the second information layer 23. A protective substrate 24 is provided on the second information layer 23 for protecting this information layer.
Examples of an optical information recording medium capable of recording and reproducing signals using laser light include a phase change type optical disk, a magneto-optical disk, an optical disk using a dye material or the like.
In a recordable phase change type optical disk, a chalcogenide typically is used as a recording layer material. Generally, the crystalline state of the recording layer material is used as a non-recorded state. Signals are recorded by irradiating the recording layer material with laser light so as to melt and quench the recording layer material so that the material becomes amorphous to form a recording mark. On the other hand, in order to erase signals, the recording layer is irradiated with laser light at a power lower than that for recording so that the recording layer becomes crystalline. The recording layer made of a chalcogenide is formed in the amorphous state. Therefore, it is necessary to crystallize entirely the recording regions beforehand to obtain a non-recorded state. In general, this crystallization for initialization is incorporated in the disk production process, and the recording layer is changed to be in the crystalline state using a laser light source or a flash light source.
For the purpose of increasing the speed at which signals are recorded on a recordable and erasable phase change type optical disk, light absorption correction that is suitable for high linear velocity has been proposed. In such a structure, for example, the light absorptance of the recording layer with respect to laser light for irradiation for recording is designed to be larger when the recording layer is crystalline than it is amorphous.
Furthermore, so-called land and groove recording in which signals are recorded both on guide grooves and on portions between the guide grooves (lands) formed on a surface of the substrate has been proposed for the purpose of improving the recording density of a recordable or recordable and erasable optical disk.
Furthermore, a one-face-double layer structure has been proposed for the purpose of increasing the recording capacity of a recordable or recordable and erasable phase change type optical disk (e.g., JP 9-212917).
In the recordable and erasable optical disk with a one-face-double layer structure, it is desired that the first information layer, which is near the side to which laser light for recording and reproduction is incident, has a high transmittance and high recording sensitivity, can be overwritten at a high speed, can be recorded both in lands and grooves, and has good repetition characteristics for recording and erasure.
Therefore, with the foregoing in mind, it is a principal object of the present invention to achieve high sensitivity of the first information layer positioned nearest the light incident side in an optical information recording medium on which signals are recorded/reproduced in an arbitrary layer by accessing at least two information layers from one side.
An optical information recording medium of a preferable embodiment of the present invention includes a substrate and a multilayered film formed on the substrate, the multilayered film comprising a first information layer, a separating layer, and a second information layer in this order from the side of the substrate. Each of the first information layer and the second information layer comprises a recording layer in which an optical change is caused by irradiation of laser light, and in the first and second information layers, signals can be recorded/reproduced on/from the recording layer by irradiating the recording layer with laser light through the substrate. The first information layer comprises a first protective layer, the recording layer, and a second protective layer in this order from the side of the substrate, and at least one of the first protective layer and the second protective layer comprises at least two layers. The at least two layers comprise a first layer and a second layer in this order from the side near the recording layer. The heat conductivity of a material constituting the second layer is larger than that of a material constituting the first layer.